我怎样才能知道我用C语言编写的应用程序打开的文件的大小? 我想知道大小，因为我想把加载文件的内容放入一个字符串，我使用malloc()分配。正在写入malloc(10000*sizeof(char));恕我直言，这是个坏主意。
当前回答
你需要搜索到文件的末尾，然后请求位置:
fseek(fp, 0L, SEEK_END);
sz = ftell(fp);
然后你可以回头寻找，例如:
fseek(fp, 0L, SEEK_SET);
或者(如果你想从头开始)
rewind(fp);
其他回答
你需要搜索到文件的末尾，然后请求位置:
fseek(fp, 0L, SEEK_END);
sz = ftell(fp);
然后你可以回头寻找，例如:
fseek(fp, 0L, SEEK_SET);
或者(如果你想从头开始)
rewind(fp);
我最终只是做了一个简短而甜蜜的fsize函数(注意，没有错误检查)
int fsize(FILE *fp){
int prev=ftell(fp);
fseek(fp, 0L, SEEK_END);
int sz=ftell(fp);
fseek(fp,prev,SEEK_SET); //go back to where we were
return sz;
}
标准C库没有这样的函数，这有点愚蠢，但我可以看到为什么它会很困难，因为不是每个“文件”都有大小(例如/dev/null)。
如何使用lseek/fseek/stat/fstat获取文件大小?
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
void
fseek_filesize(const char *filename)
{
FILE *fp = NULL;
long off;
fp = fopen(filename, "r");
if (fp == NULL)
{
printf("failed to fopen %s\n", filename);
exit(EXIT_FAILURE);
}
if (fseek(fp, 0, SEEK_END) == -1)
{
printf("failed to fseek %s\n", filename);
exit(EXIT_FAILURE);
}
off = ftell(fp);
if (off == -1)
{
printf("failed to ftell %s\n", filename);
exit(EXIT_FAILURE);
}
printf("[*] fseek_filesize - file: %s, size: %ld\n", filename, off);
if (fclose(fp) != 0)
{
printf("failed to fclose %s\n", filename);
exit(EXIT_FAILURE);
}
}
void
fstat_filesize(const char *filename)
{
int fd;
struct stat statbuf;
fd = open(filename, O_RDONLY, S_IRUSR | S_IRGRP);
if (fd == -1)
{
printf("failed to open %s\n", filename);
exit(EXIT_FAILURE);
}
if (fstat(fd, &statbuf) == -1)
{
printf("failed to fstat %s\n", filename);
exit(EXIT_FAILURE);
}
printf("[*] fstat_filesize - file: %s, size: %lld\n", filename, statbuf.st_size);
if (close(fd) == -1)
{
printf("failed to fclose %s\n", filename);
exit(EXIT_FAILURE);
}
}
void
stat_filesize(const char *filename)
{
struct stat statbuf;
if (stat(filename, &statbuf) == -1)
{
printf("failed to stat %s\n", filename);
exit(EXIT_FAILURE);
}
printf("[*] stat_filesize - file: %s, size: %lld\n", filename, statbuf.st_size);
}
void
seek_filesize(const char *filename)
{
int fd;
off_t off;
if (filename == NULL)
{
printf("invalid filename\n");
exit(EXIT_FAILURE);
}
fd = open(filename, O_RDONLY, S_IRUSR | S_IRGRP);
if (fd == -1)
{
printf("failed to open %s\n", filename);
exit(EXIT_FAILURE);
}
off = lseek(fd, 0, SEEK_END);
if (off == -1)
{
printf("failed to lseek %s\n", filename);
exit(EXIT_FAILURE);
}
printf("[*] seek_filesize - file: %s, size: %lld\n", filename, (long long) off);
if (close(fd) == -1)
{
printf("failed to close %s\n", filename);
exit(EXIT_FAILURE);
}
}
int
main(int argc, const char *argv[])
{
int i;
if (argc < 2)
{
printf("%s <file1> <file2>...\n", argv[0]);
exit(0);
}
for(i = 1; i < argc; i++)
{
seek_filesize(argv[i]);
stat_filesize(argv[i]);
fstat_filesize(argv[i]);
fseek_filesize(argv[i]);
}
return 0;
}
使用标准库:
假设你的实现有意义地支持SEEK_END:
fseek(f, 0, SEEK_END); // seek to end of file
size = ftell(f); // get current file pointer
fseek(f, 0, SEEK_SET); // seek back to beginning of file
// proceed with allocating memory and reading the file
Linux / POSIX:
可以使用stat(如果知道文件名)或fstat(如果有文件描述符)。
下面是一个stat的例子:
#include <sys/stat.h>
struct stat st;
stat(filename, &st);
size = st.st_size;
Win32:
您可以使用GetFileSize或GetFileSizeEx。
您是否考虑过不计算文件大小，只在必要时增大数组?下面是一个例子(省略了错误检查):
#define CHUNK 1024
/* Read the contents of a file into a buffer. Return the size of the file
* and set buf to point to a buffer allocated with malloc that contains
* the file contents.
*/
int read_file(FILE *fp, char **buf)
{
int n, np;
char *b, *b2;
n = CHUNK;
np = n;
b = malloc(sizeof(char)*n);
while ((r = fread(b, sizeof(char), CHUNK, fp)) > 0) {
n += r;
if (np - n < CHUNK) {
np *= 2; // buffer is too small, the next read could overflow!
b2 = malloc(np*sizeof(char));
memcpy(b2, b, n * sizeof(char));
free(b);
b = b2;
}
}
*buf = b;
return n;
}
这样做的好处是，即使对于不可能获取文件大小的流(如stdin)也可以工作。
